1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to a complementary metal-oxide semiconductor (CMOS) image sensor and a method for fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for enhancing a degree of integration of the device.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device, which converts an optic image to an electric signal. More specifically, a charge coupled device (CCD) is a device having a plurality of metal-oxide-metal (MOS) capacitors each formed within a proximate range from one another, and wherein a carrier electric charge is stored in and transmitted to each capacitor. A complementary MOS (CMOS) image sensor is a device forming a number of MOS transistors corresponding to the number of pixels by using the CMOS technology, which uses a control circuit and a signal processing circuit as peripheral circuits, and adopting a switching method, which uses the MOS transistors to sequentially detect each output.
However, the CCD is disadvantageous in that the driving method is complicated and consumes a large amount of power, and the fabrication process is complicated having too many mask process steps. Furthermore, a one-chip circuit cannot be easily formed because the signal processing circuit cannot be realized in the CCD chip. Recently, in order to overcome such disadvantages, a CMOS image sensor using sub-micron CMOS fabrication technology has been under development. Various types of pixels are used in the CMOS image sensor. However, a 3-transistor (3-T) pixel and a 4-transistor (4-T) pixel are most widely used. The 3-T pixel consists of three main transistors and one photodiode, and the 4-T pixel consists of four main transistors and one photodiode.
FIG. 1 illustrates a circuit diagram of a unit pixel of a general 4-transistor (4-T) CMOS image sensor. Referring to FIG. 1, the unit pixel of a 4-T CMOS image sensor includes a photodiode (PD), which is a photosensitive device, and four transistors (Tx, Rx, Dx, and Sx). Herein, a transfer transistor (Tx) transfers electric charge generated from the photodiode (PD) to a floating sensing node (FSN). A reset transistor (Rx) discharges the electric charge stored in the floating sensing node (FSN) for signal detection. A drive transistor (Dx) acts as a source follower, and a select transistor (Sx) is used for switching and addressing. Also, a DC gate is a supplementary transistor always applying a uniform voltage as a gate potential of the transistor, thereby allowing a uniform electric current to flow through the transistor. VDD represents a driving voltage, VSS represents a ground voltage, and output is an output voltage of the unit pixel.
Hereinafter, a related art method for fabricating a CMOS image sensor will now be described. FIGS. 2A and 2B illustrates cross-sectional views of a related art method for fabricating a CMOS image sensor.
Referring to FIG. 2A, a pad insulating layer 11 is formed on a sub-layer 10 having the photodiode and the transistors formed thereon, so as to form the unit pixel of the CMOS image sensor. Then, although not shown in the drawings, the pad insulating layer 11 is selectively removed, so as to expose a metal line within the sub-layer 10. The metal line is used for connecting the transistors to an external device.
Thereafter, a dyed photoresist is deposited and treated with light-exposing and developing processes, so as to form a color filter layer 12. Subsequently, a micro-lens planarization layer 13 is formed, so as to form a uniform micro-lens. Then, a photoresist (PR) is deposited on the micro-lens planarization layer 13, which is then treated with light-exposing and developing processes, thereby patterning the photoresist (PR).
Referring to FIG. 2B, the patterned photoresist (PR) is treated with a heat treatment to carry out a reflow process on the photoresist, thereby forming a dome-shaped micro-lens 14. Thus, the fabrication of the related art CMOS image sensor is completed.
However, the related art CMOS image sensor has the following disadvantages. The photoresist is treated with a reflow process to from the micro-lens. However, the reflow process is difficult to control, thereby causing problems in fabricating the micro-lens in a dome shape. Furthermore, as the device becomes more integrated, the radius of the curvature of the micro-lens should also be reduced, which requires the thickness of the photoresist to become thinner. However, the photoresist cannot be formed thinner than a predetermined thickness. Thus, there are limitations in reducing the radius of the curvature of the micro-lens, thereby causing difficulty in the integration of the device.